1. Field of the Invention
The present invention is related with a digital information recording apparatus for recording digital information onto an information record medium, such as a DVD-R (DVD-Recordable) which record density is about 7 times as high as that of the conventional CD-R (Compact Disc-Recordable).
2. Description of the Related Art
There is a so-called DVD as an information record medium which record density is about 7 times as high as that of the conventional CD. As one type of the DVD, there is a DVD-R of write-once-read-many type, on which the information can be written only once.
In this DVD-R, a pigment film is used for a record layer to record the information. The recording operation is performed as following. Namely, a record laser light is irradiated onto a small area portion (i.e., a portion which is to be a record pit described later) on a record track (i.e., a groove (a guide groove) or a land) which is formed in advance on the pigment film. The record laser light is focused to fit with this small area portion, and is modulated in correspondence with the digital information to be recorded. Thus, this small area portion is heated by the heat energy converted from the irradiated record laser light, so that the property such as a reflection coefficient or the like of the pigment film at this small area portion is changed to form the record pit as the property changed portion of the pigment film. In this manner, the recording operation is performed. Further, at the time of reproducing the digital information, the digital information is read out by the difference in the property such as the reflection coefficient or the like with respect to the reproduction laser light between the record pit and the portion of the pigment film, which has never been irradiated with the record laser light. At this time, in order to secure the above described large storage capacity as the DVD-R, the size of the record pit to be formed is required such that the length in the radial direction of the DVD-R (i.e. the width of the record pit) is about 0.4 .mu.m, the length in the circumferential direction of the DVD-R (i.e., the length of the record pit) is about 0.4 .mu.m for the shortest record pit and about 1.9 .mu.m for the longest record pit.
By the way, in the conventional CD-R, in case that the pigment film is used for the record layer, there is a problem that the reproduction wave form is distorted when the record pit is reproduced after the record pit is formed by the record laser light modulated by the digital information to be recorded. One of the causes for this distortion is that the shape of the record pit is not symmetrical with respect to the circumferential direction of the CD-R but is distorted in a tear drop shape, in which the record pit is thin at the front end portion thereof and is thicker at the back end portion thereof, with respect to the circumferential direction of the CD-R. Namely, as shown in FIG. 7, in case of recording the information by use of the modulation wave form of the record laser light (as shown in the top stage of FIG. 7), the attainable temperature at a position irradiated by the record laser light on the pigment film becomes low at the front end portion of the record pit and becomes high at the back end portion of the record pit (as shown in the middle stage of FIG. 7) by the heat accumulation phenomena, so that the record pit distorted in the tear drop shape is formed (as shown in the bottom stage of FIG. 7).
Another of the causes for the distortion of the record pit shape is that the light spot formed at the irradiation position of the record laser light is spewed from an area where the record pit is intended to be formed. Namely, at the time of irradiating the end portion of the area where the record pit is to be formed, the light spot is spewed out to the portion other than the area where the record pit is to be formed. As a result, the record pit is formed, which is poor in the symmetry with respect to the digital information to be recorded between the front end portion and the back end portion of the record pit.
In order to solve these problems, according to the conventional technology, the record laser light is not simply modulated by the modulation signal, in which the pulse signal is modulated by the digital information to be recorded as shown in the top stage of FIG. 7, but the wave form conversion is further applied to the wave form after the modulation by use of the digital information to be recorded, so that the record signal corresponding to the digital information to be recorded is obtained. Then, the record laser light is modulated by use of this record signal after the wave form conversion, so as to form the record pit.
That is to say, as shown in FIG. 8, the modulation signal (as shown in the top stage of FIG. 8), which is modulated by the digital information to be recorded is further wave-form-converted, so that a first portion of the wave form corresponding to the back end portion of the record pit is erased for a predetermined interval, and that a next portion of the wave form corresponding to the front half portion of the record pit is constructed by the continuous short pulse waves to form the record signal (as shown in the bottom stage of FIG. 8). In the example shown in FIG. 8, as for the signal having a length of 11 T (T is a length corresponding to one standard clock of the digital information to be recorded, and it is standardized that the digital information to be recorded on the DVD-R is constructed by a combination of 9 kinds of data pulses i.e., from a pulse having a length of 3 T to a pulse having a length of 11 T, and a synchronization pulse having a length of 14 T), the first portion is erased by an amount of 1.5 T, and the latter half portion is wave-form-converted such that the pulses each having a length of 0.5 T are continuous, so that the record signal as shown in the bottom stage of FIG. 8 is generated. Then, the record laser light is modulated by use of the record signal generated in this manner and is irradiated onto the pigment film. By modulating the record laser light by the wave-form-converted record signal in this manner, the temperature change in the pigment film is not as shown in the middle stage of FIG. 8, but it is possible to form the record pit in an oval shape better in the symmetry corresponding to the digital information to be recorded.
Here, in order to wave-form-convert the modulation signal shown in the top stage of FIG. 8 to the record signal shown in the bottom stage of FIG. 8, a wave form conversion circuit equipped with at least a delay circuit is required. Namely, it is possible to erase the first portion of the modulation signal by an amount of 1.5 T for example, by calculating a logical product of a delay signal, which is generated by delaying the modulation signal by a delay circuit by an amount of 1.5 T, and an original wave form.
Further, it is possible to convert the latter half portion of the modulation signal to the wave form, in which the short pulses are continuously arranged in time sequence as shown in FIG. 8, by operating a flip-flop circuit by use of a standard clock as a trigger on the basis of the modulation signal, and calculating the logical product of the output of the flip-flop circuit and a delayed clock signal, which is obtained by delaying the standard clock for a predetermined delay time period.
In this manner, the delay circuit is required in either case.
By the way, as this kind of delay circuit, there are an analog type delay circuit using a delay line, and a digital type delay circuit. Among them, the former may be constructed by a lumped parameter element, a distributed parameter element, an active element such as a logical gate, and so on. The latter may be constructed by shift registers in predetermined stages (=predetermined delay time period/cycle of standard clock), a counter for counting a predetermined number of standard clocks, and so on. Desirable one of these methods is determined mainly in consideration with various factors such as the required delay time period, the required accuracy, the possibility of realization, and so on. Further, as the occasion demand, a delay line may be utilized which can make the delay time period variable.
On the other hand, in case of the conventional CD-R, the standard clock cycle is about 230 nsec (nanoseconds), so that about an accuracy of 10 nsec is necessary and enough as an accuracy for the above mentioned wave-form-conversion from the modulation signal to the record signal. Thus, it is possible to construct the wave-form-conversion circuit by use of the conventional delay line without any difficulty.
However, in case of the DVD-R, the standard clock cycle is about 38 nsec so as to realize the large storage capacity, and a very high accuracy for the above mentioned wave-form-conversion is required. More concretely, a resolution and a repeatability in the accuracy of about a few nsec (e.g., 1 nsec if possible) is required. Further, it is also required to enable changes in the various settings (e.g., the change in the wave form to be converted) with such a high accuracy.
However, the delay line available as the conventional delay circuit has the resolution of about 5 nsec. And that, it is necessary, in case of the delay line, to consider the accuracy error with respect to the total delay time period and the change in the delay time period during the actual usage in the range of about .+-.10%. Furthermore, in consideration with the dispersion in the delay time periods between the delay lines connecting the respective elements, the substantial accuracy is even more degraded, which is a problem in this case.
On the other hand, in case of the digital type delay circuit, it is necessary to use the clock signal having the cycle of about 1 GHz in order to achieve the resolution of about 1 nsec. However, the cycle of the clock signal available in the general CMOS (Complementary Metal-Oxide-Semiconductor) logical circuit except for a very high speed logical element such as a so-called ECL (Emitter Coupled Logic) circuit or the like, is limited to be as much as 100 MHz. As a result, the attainable resolution is about 10 nsec, which is a problem in this case of the digital type delay circuit.
Further, in the construction in which the delay time period is variable, it is necessary to connect a large number of special delay lines of high resolution type in concatenation. Thus, the resolution as a whole is certainly degraded due to the dispersion of the respective delay lines, which is a problem in this case.
Furthermore, if utilizing the above mentioned ECL circuit or the delay lines of high resolution type in order to realize the delay circuit having the high resolution, the structure of the circuit becomes complicated and the power consumption is increased. And that, the hardware circuit itself would be quite expensive.